docs: Update function "run" to "invoke" in fake_llm.ipynb (#19570)

This patch updates function "run" to "invoke" in fake_llm.ipynb. Without
this patch, you see following warning.

LangChainDeprecationWarning: The function `run` was deprecated in
LangChain 0.1.0 and will be removed in 0.2.0. Use invoke instead.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>

cookbook/fake_llm.ipynb

@@ -100,7 +100,7 @@
     }
    ],
    "source": [
-    "agent.run(\"whats 2 + 2\")"
+    "agent.invoke(\"whats 2 + 2\")"
    ]
   },
   {